Method for continuously melting down scrap metal

ABSTRACT

In a process for the continuous melting down of scrap metal, in particular of bales of scrap metal, which contain residual plastic materials, the scrap metal being fed to a melt-down reactor (1) and the heat required to melt this down being obtained by direct heating from below, using burners that are arranged close to the tapping point, the process is managed in the melt-down reactor (1) as a reducing process; part of the process gases is returned to the melt-down reactor (1) as combustion gas after utilization of the sensible heat and purification; the remaining portion is passed out as output gas. This makes it possible to operate without oversize gas purification systems when melting down scrap metal that contains plastic, and to utilize the pyrolysed substances that are contained in the working material to good effect.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for continuously melting downscrap metal, in particular, bales of scrap metal containing residualplastic materials, the scrap metal being fed to a melt-down reactor andthe heat required to melt the scrap being provided by direct heatingfrom below, using burners that are located near the tapping point.

2. Description of the Related Art

The use of a shaft furnace to melt down scrap metal, in which the columnof material that is to be melted is heated from below by burners, isalready known from DE-PS 25 04 889. The fuel-oxygen combustion that isused in the shaft furnace for direct heating is so managed that areducing atmosphere area is created beneath the oxidizing melt-down zoneby adding large pieces of coal to the material to be melted. As a rule,relatively large amounts of plastic materials are contained in thematerial to be melted down, particularly when utilizing types of scrapmetal that contain organic materials, such as occur, for example, whendisposing of motor vehicles, or in the case of material that is to bemelted down and which has been obtained by compressing bales of scrapmetal, for example, from automobiles, and these lead to a relativelyhigh concentration of injurious substances in the waste gases.

Considerable quantities of gas that contain harmful organic or inorganicsubstances are generated by heating melt-down reactors by burners,particularly in the case of the material that is to be melted down asreferred to herein, so that large-scale and costly gas purifyingfacilities are required.

SUMMARY OF THE INVENTION

It is the aim of the present invention to so develop a method of thetype described in the introduction hereto that scrap metal that containsplastics, in particular bales of scrap metal of the kind that resultfrom the disposal of motor vehicles, can be melted down without the needto provide oversize gas purifying facilities. At the same time, thepresent invention is aimed at utilizing the pyrolysed substancescontained in the material to be melted down to good effect and tominimize the energy and technical costs of the system accordingly. Inorder to do this, the method according to the present invention isessentially such that the process in the melt-down reactor is managed asa reducing process; such that some of the process gases are returned tothe melt-down reactor as combustion gas after utilization of thesensible heat and purification; and such that the remainder is passedout as output gas. The reducing process in the interior of the melt-downreaction ensures that a process gas that contains a high proportion ofchemically bound heat in addition to sensible heat is made available.The reducing management within the melt-down reactor offers thepossibility of obtaining a combustion gas as process gas, which can beused subsequently as a substitute for the combustion gases that are usedduring the start-up phase of the melt-down reactor. Appropriatemanagement and selection of the material to be melted down permitsautothermic conduct of the process, a reducing atmosphere management ofthe process within the melt-down reactor then permitting a type ofcomplete autothermic process management if, as in a preferreddevelopment, types of scrap metal with a chemically bound energy contentof >0.6 kWh.t⁻¹ are used as the working material in the melt-downreactor.

A further improvement of the energy balance can be obtained in that thesensible heat of the waste gases is used to pre-heat the combustiongases that are returned to the melt-down reactor. This type ofpre-heating of the returned combustion gases makes it possible to uselow-quality combustion gas to maintain the desired process parameterswithin the interior of the melt-down reactor.

Plant-engineering are considerably reduced with a simultaneous reductionof energy consumption are considerably reduced in that more than20%-vol. of the process gas that is drawn off is to the melt-downreactor as heated combustion gas after purification, in particular afterHCl-scrubbing and SO₂ -/H₂ S separation the remaining part being drawnoff as output gas.

The remaining part of the process gas is drawn off as output gas. Aprocess of this kind in a circulating system makes it possible toscale-down the systems for purifying the process gases to a considerableextent, when the method can be so regulated that the CO content of theprocess gases from the melt-down reactor can be adjusted to 30 to40%-vol., in particular, approximately 36%-vol. by thesub-stoichiometric introduction of 0₂ gas. This kind of management ofthe process, with a CO content between 30 and 40%-vol. makes the processgas or combustion gas, which is distinguished by a thermal value of 1.5to 2 kWh.m⁻³ n, available for an autothermic process.

Thus, it is an advantage that the process can be so managed that theoutput gas, which exceeds the amount of process gases that are returned,can be generated with a thermal value of 1.5 to 2 kWh.m⁻³ n afteradsorptive purification. A thermal value of this kind permitsproblem-free return to the melt-down reactor and makes it possible tominimize the energy consumption with respect to additional primaryenergy carriers.

In order, in particular, to obtain an output gas with an even highervalue, or to permit autothermic operation if the working substance has achemically bound energy content that is too low, the procedure can,advantageously, be so managed that the process gas, in particular theoutput gas, is adjusted to a thermal value of 3 to 3.5 kWh.m⁻³ n inparticular 3.3 kWh.m⁻³ n over a coke-bed reactor.

Particularly advantageous purification of the waste gases from theprocess can be achieved in that the process gases passed over a mixedbed adsorber that uses coke and lime and the charged adsorbent isreturned to the melt-down reactor. This ensures that the removal of thesulphur that has been introduced into the system is effected by way ofthe slag so that, in particular, the undesirable production of surplusand additional secondary products, such as, for example, gypsum thatcontains chloride and fluoride, can he eliminated.

Simple and effective purification of the waste gases from the processcan be effected in that the waste gases from the process are passedthrough a coarse separator, preferably a hot-gas cyclone-type separatorto a thermal reactor and then to a heat exchanger for heating thecombustion gases that are to be returned in the circulating system, inwhich connection the process gas can be used before physical andchemical purification, after pre-heating of the combustion gases thathave been returned, in order to generate process steam. Processmanagement of this type makes it possible to hold the product or processgas that is drawn off at the head of the melt-down reactor in a thermalreactor, after coarse dust removal, for a desired and sufficiently longreaction time in order to ensure the desired chemical conversion and, inparticular, establishment of equilibrium, after which the sensible heatcan be used in a heat exchanger, in order to pre-heat the combustion gasthat is returned.

In a subsequent gas purification stage, dust that results from theprocess can be removed in the conventional manner by means ofelectro-filters and then hydrogen chloride/hydrogen fluoride can bewashed out, and a combustion gas that has been largely freed from thehydrogen halides is made available for return as a melting energycarrier or for use as an output gas after subsequent H₂ S and SO₂adsorption.

The burners that are located close to the tapping point of the melt-downreactor are operated, at least in part, with the purified process gases,when, advantageously, 20 to 30%-vol. of the purified process gas isdelivered to the burners. The process gas that is circulated isadvantageously pre-heated to a temperature between 500° and 700° C., andpreferably to a temperature of approximately 600° C., by counter-flowheating with the process gases prior to being delivered to the burners;this reduces the cost for producing the required melting heat to thepoint that an external supply of combustion gases, for example, naturalgas, can be eliminated after the start-up phase.

The present invention is described in greater detail below on the basisof one embodiment that is shown diagrammatically in the drawing appendedhereto and which is used for carrying out the process according to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT

The drawing shows a melt-down reactor 1, to which, in addition tometallic materials 2, for example scrap metal, which contains residualplastic materials, coke 3 and lime 4 are also added. Slag 5 as well asmolten metal 6 are drawn off from the melt-down reactor 1 in its lowerarea. Possible carburization of the metallic smelt to a carbon contentof approximately 3% requires the addition of coke at approximately 60kg.t⁻¹ (relative to the molten metal); this can, however, be varied as afunction of the metallic material that is being used.

The heat necessary to melt the metal within the melt-down reactor 1 isapplied by direct heating from below in the lower area, using theburners 7 which, after the start-up phase, are supplied through the line8 with process gases that are returned and purified together with anoxygen feed 10. The supply of oxygen (calculated at 90% purity) iseffected at a temperature of approximately 100° C. For the start-upphase, or in the event that the thermal value of the process gases thatare returned is insufficient for autothermic operation, additionalcombustion gas, for example natural gas, is supplied to the burners, asindicated diagrammatically in the drawings at 11.

Process gas is drawn off at the head of the melt-down reactor 1 througha line 12 and passed to a coarse separator, preferably a hot-gascyclone-type separator 13, in which primary dust separation is carriedout, whereupon the separated dust being returned to the melt-downreactor along the line 14.

The process gas passes from the hot-gas cyclone-type separator 13 into athermal reactor 15 and is passed at a temperature of approximately1,000° C. from this to a multi-stage heat exchanger 16, in which, in afirst stage 17, the combustion gas that is to be returned and purified,which passes along the line 8 to the burner 7, is pre-heated from atemperature of approximately 90° C. to approximately 600° C. in acounter-flow heat exchanger. The process gas, which is essentiallyunpurified, is cooled from 1,000° C. to approximately 900° C. when thisis done. In a subsequent waste-heat boiler 18, a major part of thesensible heat of the process gas is used to generate process steam, asis indicated at 19, when the gases are cooled to approximately 300° C.In a further circulating system 20, formed, for example, as a thermo-oilheat exchanger, the process gas is cooled even more before being passedto an electro-filter 21 in which, once again, dust 22 is separated out.Next, the process gas passes to an HCl scrubber 23 in which 95% of thehydrogen chloride and hydrogen fluoride are separated out. 30% or 50%caustic soda solution 25 is added to a waste water purification plant 24in order to purify the waste water from the HCl scrubber 23, and NaClgranulate 26 as well as filter cake 27 can be removed from thispurification plant.

After the HCl scrubber 23, the process gas is pre-heated to atemperature of approximately 90° C. in the heat exchanger 20 and thencompressed in a fan 28 before it is passed to a travelling bed adsorber.In particular, the separation of H₂ S, SO₂, as well as the residues ofHCl and HF, takes place in this travelling bed adsorber. The adsorbentthat is drawn off from the travelling bed adsorber 29, which is formedfrom lime and coke and indicated at 30 is returned to the melt-downreactor 1 along the line 31. The coke and lime that are used in thetravelling bed adsorber or mixed bed adsorber 29 are numbered 32 and 33.By returning the adsorbent 30 that is charged with sulphur to themelt-down reactor, most of the sulphur is bound in the slag, which meansthat no waste or by-products such as REA gypsum, which contain sulphur,are generated in the gas purification process.

The purified process gas that is drawn off from the adsorber 29 is inpart, for example, in a quantity of somewhat more than 20%-vol.,compressed once more in an additional fan 34 and, as indicated above,pre-heated to approximately 600° C. in a counterflow heat exchangerbefore it is returned to the melt-down reactor once again by way of theburners 7.

The portion of the purified process gas that is not returned to themelt-down reactor is drawn off at 35 as output gas.

Taken all in all, apart from a certain start-up phase, the process runsto a large extent autothermically in that some of the purified reductionor process gas is returned, so that there is no requirement to useadditional combustion gases. Thus, the energy required to producemetallic smelts in the melt-down reactor 1 is extracted in part from theorganic fraction contained in the metallic working substance or scrapmetal and, in part, from the coke that cannot be used for thecarburization because of the carbon consumption reactions. The thermalvalue of the output gas that is produced is dependent, amongst otherthings, on the organic waste or plastic fraction in the scrap metal thatis used. In the case of a scrap metal that consists of baled motorvehicles with an organic fraction of approximately 20%, the thermalvalue of the process or export gas that is produced by the processdescribed herein is approximately 2.0 Kwh.m⁻³ n. In order to elevate thethermal value of the export gas, this can be passed over a coke-bedreactor, which is not described in greater detail herein.

The process in the melt-down reactor is carried on in a reducingatmosphere, i.e., sub-stoichiometrically, in such a way that on leavingthe reactor the output gas has a CO content of approximately 30 to40%-vol., preferably, approximately 36%-vol. Altogether, both thesensible and the chemically bound heat of the process gas is utilized,the utilization of the chemically bound heat being effected through thereturn of waste gas to the melt-down process. The thermal utilization iseffected by combustion of the returned gases with a simultaneous oxygenfeed to the burners 7.

We claim:
 1. A process for continuously melting down scrap metal whichcontains residual plastic materials comprising:feeding scrap metal to bemelted down to a melt-down reactor; supplying heat to melt down thescrap metal by direct heating from below using burners arranged close toa tapping point of the reactor; controlling operation of said reactor tobe a reducing process; drawing process gases off from the reactor; usingexcess heat carried by said process gases; purifying said process gases;returning a portion of said process gases to the melt-down reactor as acombustion gas; and venting a remaining portion of said process gases asan output gas.
 2. A process as in claim 1 wherein said step of purifyingcomprises HCl scrubbing and SO₂ -/H₂ S separation and wherein said stepof returning comprises returning more than 20% vol. of said processgases.
 3. A process as defined in claim 1 or claim 12, wherein said stepof using said excess heat comprises using said excess heat to pre-heatsaid portion of the process gases returned to the melt-down reactor. 4.A process as defined in claim 1, further comprising adjusting a COcontent of the process gases from the melt-down reactor to about30-40%-volume, by a sub-stoichiometric supply of O₂.
 5. A process as inclaim 4, wherein said step of adjusting comprises adjusting the COcontent to approximately 36%-vol.
 6. A process as in claim 1 or claim14, wherein at least said remaining portion of said process gases has athermal value of 1.5 to 2 kWh.m⁻³ n following said purification step. 7.A process as in claim 1 or claim 14, further comprising adjusting athermal value of at least said remaining portion of said process gasesto 3.0 to 3.5 kWh.m⁻³ n, over a coke-bed reactor.
 8. A process as inclaim 7, wherein said step of adjusting said remaining portion of saidprocess gases comprises adjusting to a thermal value of 3.3 kWh.m⁻³ n.9. A process as in claim 1, wherein said step of feeding scrap metalcomprises feeding scrap metal with a chemically bound energy contentgreater than 0.6 kWh.t⁻¹.
 10. A process as in claim 1, furthercomprising passing said process gases over a coke-bed reactor thatoperates with coke and lime and returning a charged absorbent to themelt-down reactor.
 11. A process as in claim 1 wherein said step ofpurifying comprises passing the process gases through a coarse separatorto a thermoreactor and subsequently to a heat exchanger to heat saidprocess gases.
 12. A process as defined in 11 wherein said step ofpassing through a coarse separator comprises passing through a hot-gascyclone-type separator.
 13. A process as in claim 1, wherein said stepof returning a portion of the process gases includes preheating saidportion of said process gases to a temperature of between about 500-700°C. prior to entering burners of said reactor by counterflow heating saidportion of said process gases with said process gases drawn off fromsaid reactor.
 14. A process as defined in claim 13 wherein said step ofpreheating said portion of said process gases comprises preheating toapproximately 600° C.